Synchronized rhythmic discharges of the secondary olfactory neurons in carp

(1) Rhythmic and synchronized multiunit bursts of spike discharges, which were superimposed on the peaks of the oscillatory potentials of 4--10 Hz (olfactory tract waves), were induced in the olfactory tract fibers of the carp in response to olfactory stimulation. Suppression of the spike discharges was observed during the interburst periods of discharges. (2) Single-unit analysis revealed three types of discharge patterns in the olfactory tract fibers in response to olfactory stimulation: rhythmic excitation (Type 1), suppression (Type 2) and no response (Type 3). A synchronization of spike discharges was observed among the Type 1 units. (3) Mass potentials were evoked in the olfactory tract by electrical stimulations applied to the olfactory nerve. When the time interval between the conditioning stimulation and the test stimulation was about 200 msec, a transient recovery of the evoked mass response was observed. This interval approximately corresponded to the cycle length of the olfactory tract waves. (4) It was concluded from the results obtained that the olfactory tract waves are the summated action potentials from the axons of the secondary olfactory neurons discharging rhythmically and synchronously. (5) An inhibitory phasing mechanism in the olfactory bulb was suggested which may cause the synchronized rhythmic activity of the secondary olfactory neurons.